Distributed transcoding

ABSTRACT

A method of delivering a video stream, including encoding a video stream into a first encoded stream and selecting encoding parameter values for subsequent transcoding the first encoded stream into a specific second encoded stream. Furthermore, the method includes transmitting the first encoded stream with the selected encoding parameter values to a transcoder over a standard communication network, transcoding the first encoded stream into a second encoded stream having the selected parameter values and forwarding the second encoded stream.

FIELD OF THE INVENTION

The present invention relates to communication systems and in particularto systems for delivery of video signals.

BACKGROUND OF THE INVENTION

One of the usages of communication networks, such as cable television,the Internet and the like, is the distribution of movies in the form ofvideo streams. Video streams are formed from a sequence of frames (i.e.,still images), with corresponding display times. The transmission ofvideo streams requires wide bandwidth and therefore video streams aregenerally compressed before transmission. Various compression methods,also referred to sometimes as encoding methods, have been defined. Onepopular compression technique is the MPEG-2 compression method. Anothertechnique, generally considered to give better compression ratios is theH.264 method.

In some cases it is desired to change the compression of a video streamalong a path from a server to a client, for example in order to match itwith the capabilities of the client and/or with the network conditions.Such changing of the compression is often referred to as transcoding.

US patent publication 2007/0121651 to Casey et al., published May 31,2007, the disclosure of which is incorporated herein by reference,describes a network based converter, which is adapted to convertmultimedia signals between various formats, for example between MPEG-2and H.264 formats, on the fly.

The compression of video streams typically involves a first stage ofidentifying information, of a sufficient quantity, which can be droppedwith minimal loss to the image quality and a second stage in which thecompression is performed based on the information from the first stage.Depending on the compression method, in the first stage, parametervalues indicating how each frame is to be represented in the compressedform are selected. For example, quantization parameter values indicatehow many bits are to be used to represent the data of each frame or ofportions thereof.

The first stage of the compression is generally processing powerintensive, and requires expensive hardware for real time, on-the-flycompression. Cheaper hardware can sometimes be used by selectingcompression parameter values from a smaller set of possibilities, butthis may result in less efficient compression.

FIG. 1 is a schematic illustration of a video delivery system 100 of theprior art. An encoder 102 prepares a compressed video stream inaccordance with a first compression format. The compressed video streamis transmitted over a network 108 to a transcoder 110, which transcodesthe video stream into a second size and/or format, for transmission to aclient 112. Transcoder 110 generally comprises a decoder 120 whichpartially or entirely decodes the compressed video stream; a parameterselector 122, which selects parameter values for the transcoding; and anencoding engine 124, which performs the actual compression.

U.S. Pat. No. 6,542,546 to Vetro et al., titled: “Adaptable compressedbitstream transcoder”, the disclosure of which is incorporated herein byreference, describes a video transcoder adapted to operate in aplurality of transcoding modes. A manager of the transcoder dynamicallyselects one of the transcoding modes to be used, according to thecontent of the video stream transcoded and the network conditions.

US patent publication 2005/0232497 to Yogeshwar et al., published 20Oct. 2005, the disclosure of which is incorporated herein by reference,describes a transcoding technique for translating from a firstcompression format to a second compression format. The transcodingtechnique uses compression parameters of the first format in selectingcompression parameters of the second format. In one embodiment, thetranscoding is performed from MPEG-2 to H.264.

US patent publication 2001/047517 to Christopoulos et al., publishedNov. 29, 2001, the disclosure of which is incorporated herein byreference, describes a transcoding method in which a video stream isprovided to a transcoder with transcoding hints. The transcoder encodesthe video stream to fit onto a channel leading to the client, based onclient capabilities and link characteristics of the channel, using theprovided hints.

The methods referred to hereinabove may reduce the complexity of thetranscoder to some extent, but due to the increasing complexity ofadvanced compression methods, transcoders are still quite complex andexpensive.

Other patent publications which may be of interest include US patentpublication 2005/0276580 to Zacek, published 15 Dec. 2005, EP patentapplication 1,069,770 to Tomson Licensing S.A., published 17 Jan. 2001,EP patent application 1 439 707 to Broadcom Corporation, filed 19 Jan.2004, and US patent publication 2007/0147440 to Song et al., publishedJun. 28, 2007, the disclosures of all of these are incorporated hereinby reference.

SUMMARY OF THE INVENTION

An aspect of some embodiments of the present invention relates to amethod of transcoding a video stream in which a transcoder transcodesvideo streams using parameter values received with the video streams.

In some embodiments of the invention, parameter values are received forat least 90% of the video stream or even for at least 99% of the videostream. Optionally, the received parameter values relate to one or moreof motion vectors, quantization values and macro-block type. Optionally,the received parameter values are derivable from the received videostream, but are received with the video stream in order to reduce theprocessing resources required by the transcoder. Alternatively, thereceived parameter values are derived from a higher quality (e.g.,original) version of the received video stream before it was compressed,and derivation of the parameters from the received video stream willresult in a lower quality transcoding.

In some embodiments of the invention, the transcoder receives parametervalues for substantially all the parameters required for encoding, suchthat the transcoder does not select parameter values, at least for 90%or even at least for 99% of the video stream. Reducing or even removingthe parameter value selection task of the transcoder makes thetranscoder simpler, albeit perhaps at the cost of reducing or evenremoving the ability of the transcoder to adjust the transcoding to thenetwork and/or client capabilities. If necessary, an additionaltranscoding stage may be used to adjust the transcoded video stream tonetwork or client conditions.

In some embodiments of the invention, the transcoding includes changinga compressed video stream from a first compression format to a secondformat. The second compression format is optionally less compact thanthe first compression format, for equivalent quality, on the average.

The parameter values are optionally supplied to the transcoder inheaders of the packets of the compressed format.

Supplying the parameter values with the video stream to the transcoderallows central selection of the parameter values, such that the cost ofthe hardware required for performing the selection is much lower thanthat required for employing strong compression units in each transcodinglocation. In addition, the supplying of the parameter values from acentral location allows precise control of the quality of the videostream reaching the clients.

An aspect of some embodiments of the invention relates to a method ofvideo stream transcoding by a transcoder, in which motion vector valuesare identified in advance and transmitted to the transcoder, rather thanbeing identified by the transcoder.

There is therefore provided in accordance with an exemplary embodimentof the invention, a method of delivering a video stream, comprisingencoding a video stream into a first encoded stream, selectingcompression parameter values for transcoding of the first encoded streaminto a specific second encoded stream, transmitting the first encodedstream with the selected compression parameter values to a transcoderover a standard communication network, transcoding the first encodedstream into a second encoded stream having the selected parameter valuesand forwarding the second encoded stream.

Optionally, transmitting the first encoded stream comprises transmittingover a distance of at least 1 kilometer. Optionally, transmitting thefirst encoded stream comprises transmitting to a plurality oftranscoders. Optionally, transmitting the first encoded stream comprisestransmitting over an Ethernet, IP cable or satellite network.Optionally, transcoding the first encoded stream into a second streamcomprises decoding the first encoded stream into a non-compressed streamand re-encoding the non-compressed stream into the second encodedstream. Optionally, the second encoded stream is larger than the firstencoded stream by at least 20%

In some embodiments of the invention, the first and second encodedstreams differ in the resolution of the video stream and/or in theircompression method. Optionally, transcoding the first encoded streaminto a second encoded stream comprises transcoding an H.264 stream intoan MPEG-2 stream. Optionally, transmitting the first encoded stream withthe selected compression parameter values comprises transmitting withall parameter values required for generating the second encoded stream.Alternatively or additionally, transmitting the first encoded streamwith the selected compression parameter values comprises transmittingwith fewer than all the parameter values required for generating thesecond encoded stream.

Optionally, transmitting the first encoded stream with the selectedcompression parameter values comprises transmitting with motion vectorvalues. Optionally, transmitting the first encoded stream with theselected compression parameter values comprises transmitting values forone or more specific parameters, for more than 90% of the video stream.Optionally, the method includes transcoding the second encoded streaminto a third encoded stream. Optionally, transcoding the second encodedstream into a third encoded stream comprises transcoding after theforwarding of the second stream. Optionally, transcoding the secondencoded stream into a third encoded stream comprises at least partiallydecoding and re-encoding the stream or even decoding the second streaminto a non-compressed stream and then encoding into the third encodedstream.

There is further provided in accordance with an exemplary embodiment ofthe invention, a method of delivering a video stream, comprisingreceiving, over a standard communication network, a first encoded videostream together with compression parameter values, transcoding the firstencoded stream into a second encoded stream having the compressionparameter values and forwarding the second encoded stream.

Optionally, receiving compression parameter values comprises receivingvalues of at least one compression parameter for at least 80% or even95% of the video stream. Optionally, receiving compression parametervalues comprises receiving values of at least three compressionparameters for at least 80% of the video stream.

There is further provided in accordance with an exemplary embodiment ofthe invention, a transcoder, comprising a network interface and aprocessor configured to receive an encoded video stream through thenetwork interface, together with compression parameter values and totranscode the received encoded video stream into a different encodedvideo stream having the received compression parameter values.

Optionally, the processor is configured to decode the received encodedvideo stream into a non-compressed video stream and re-encode thenon-compressed video stream in transcoding the received encoded videostream.

Optionally, the processor is configured to generate the differentencoded video stream substantially entirely according to receivedcompression parameter values and without selection of parameter values.

Optionally, the processor is configured to generate the differentencoded video stream with received parameter values of a plurality ofdifferent parameters for at least 90% of the video stream.

There is further provided in accordance with an exemplary embodiment ofthe invention, a video server, comprising an input interface forreceiving video streams, an encoder configured to compress video streamsreceived through the input interface according to a first compressionformat, a parameter selector configured to select values for one or moreparameters of a second compression format, for the received videostreams and an output interface configured to transmit compressed videostreams with corresponding parameter values toward a transcoder.

Optionally, the parameter selector is configured to select only a singlevalue for at least 95% of the parameter instances for which it selectsvalues. Optionally, the parameter selector is configured to select onlya single value for at least 99% of the parameter instances for which itselects values. Optionally, the output interface is configured totransmit compressed video streams with corresponding parameter values,including only a single value for at least one compression parameter,for at least 99% of the video stream.

BRIEF DESCRIPTION OF FIGURES

Exemplary non-limiting embodiments of the invention will be describedwith reference to the following description of embodiments inconjunction with the figures. Identical structures, elements or partswhich appear in more than one figure are preferably labeled with a sameor similar number in all the figures in which they appear, in which:

FIG. 1 is a schematic illustration of a video delivery system of theprior art;

FIG. 2 is a schematic illustration of a video delivery system, inaccordance with an exemplary embodiment of the invention;

FIG. 3 is a schematic block diagram of a compression parameter selector,in accordance with another exemplary embodiment of the invention; and

FIG. 4 is a block diagram of a video distribution system, in accordancewith another exemplary embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS Network Overview

FIG. 2 is a schematic illustration of a video delivery system 200, inaccordance with an exemplary embodiment of the invention. System 200includes a video processing and storage unit 250 (referred to herein asP&S unit), which compresses and transmits video streams. P&S unit 250optionally includes an encoder 102 which is configured to receive videostreams and compress them for transmission, for example in accordancewith the H.264 compression method. A parameter selector 222 prepares forthe compressed video stream, parameter values to be used in transcodingthe compressed video stream into a second compression format, forexample the MPEG-2 format. The compressed video stream and the parametervalues are optionally combined by a combiner 230 into a single stream,for storage and transmission. The combined video stream may be stored ina storage unit 204 for later use or may be provided directly to a server232, typically for real time transmission. Server 232 transmits thecombined video stream toward one or more clients 112 over a network 108.

A transcoder 210 optionally receives the combined video stream andconverts it into a video stream in accordance with the secondcompression format, for one or more clients 112 configured to receivevideo streams in the second format. Transcoder 210 optionally comprisesa decoder 120, which decodes the compressed video stream of the firstcompression format, and an encoding engine 124 which recompresses thevideo stream into the second compression format, according to theparameter values received in the combined video stream from parameterselector 222. Unlike prior art transcoder 110 (FIG. 1), which includes aparameter selector 122, transcoder 210 optionally does not performparameter selection, but rather operates based on parameters selected byparameter selector 222 of P&S unit 250.

In some embodiments of the invention, the transcoded video stream fromtranscoder 210 is provided directly to a client 112 without furthertranscoding. Alternatively or additionally, for some or all of theclients, the transcoded video stream from transcoder 210 is passed to arate transcoder 240, which adjusts the size and/or rate of thetranscoded video stream to the capabilities of a transmission channel tothe client 112. It was determined by the inventor of the presentinvention, that the double transcoding by transcoders 210 and 240 is inmany cases cheaper and/or provides higher quality video data than asingle transcoder 110 of the prior art.

Clients 112 supporting the first compression format are optionallyprovided the combined stream directly from server 232 through network108. Optionally, the combined stream is configured such that clientssupporting the first compression format view the parameter values aspadding which is ignored. Alternatively or additionally, one or moreclients supporting the first format receive the first compression formatstream through a statistical multiplexer (Stat Mux) 234 or other networkelement, which extracts the compressed stream from the combined stream,for the client 112. In some embodiments of the invention, one or moreclients 112 receive the video stream through prior art transcoders 110which are not adapted to use the parameter values supplied in thecombined streams. The transcoder 110 optionally ignores the parametervalues in the combined stream and performs the transcoding using anymethod known in the art. Thus, the advantages of embodiments of thepresent invention described above may be achieved without replacing allthe transcoders in the network.

Video Processing and Unit

Processing and storage unit 250 is optionally located entirely in asingle location, possibly having some or all of its elements mounted ona single computer system. Alternatively, processing and storage unit 250is distributed over two or more nodes of the network, with differentelements thereof in different locations. Further alternatively, some ofthe elements shown above as belonging to P&S unit 250 are notimplemented at all. For example, a unit which only operates with realtime video streams may be used, in which case storage unit 204 is notrequired. In an exemplary embodiment of the invention, the encoding andparameter selection are performed in a first location, while storageunit 204 and server 232 are situated and implemented at a secondlocation, possibly at least a kilometer or even at least 100 kilometersaway from encoder 102. In some embodiments of the invention, storageunits 204 are positioned near transcoder 210, rather than at P&S unit250. In some embodiments of the invention, a plurality of storage units204 and/or servers 232 are employed to service clients in differentgeographical locations; the storage units 204 and/or servers 232receiving combined video streams from a single set of combiner 232,encoder 102 and parameter selector 222. In another exemplary embodiment,encoder 102 is distanced from parameter selector 222.

FIG. 3 is a schematic block diagram of parameter selector 222, inaccordance with an exemplary embodiment of the invention. In theembodiment shown in FIG. 3, parameter selector 222 comprises a decoder302 which decodes the compressed video stream from encoder 102 into anon-compressed form. The decoded non-compressed form of the videostream, passes from decoder 302 to a second format encoder 304, whichcompresses the decoded non-compressed form of the video stream inaccordance with a different compression format. Second format encoder304 is optionally a high performance encoder, which uses any of themethods known in the art to achieve a high quality compression inaccordance with the second compression format. Rather than using thecompressed result, parameter selector 222 collects the parameters usedin generating the compressed result and these parameters are combined tothe compressed video stream of the first format, by combiner 230 (FIG.2). In FIG. 3, which is suitable for example in cases in which theMPEG-2 format is the second format, the collection of parameters isperformed simply by a header collector 306 which collects the headers ofthe video units (e.g., frames, macro-blocks) of the compressed videostream, as these headers include the determined parameter values. Insome embodiments of the invention, the headers themselves are combinedto the the compressed video stream of the first format, by combiner 230.

As shown, second format encoder 304 operates on a compressed (by encoder102) and decompressed (by decoder 302) version of the video stream, sothat the selection of parameters will be in the same conditions as thecompression of encoding engine 124 is performed. Alternatively, theparameter selection is performed on the original video stream beforedata is lost due to compression. Optionally, parameter selector 222 doesnot filter or otherwise preprocess the video stream so that the videostream on which the selection is performed is identical to the stream onwhich encoding engine 124 operates. Alternatively, parameter selector222 slightly preprocesses the video stream so as to enhance theencoding, while leaving the stream on which the preprocessing isperformed very close to the stream on which encoding engine 124operates. Alternatively or additionally, the parameter values providedby parameter selector 222 include the preprocessing tasks whichtranscoder 210 subsequently performs on the video stream after it isdecoded, before it is recompressed by encoding engine 124.

In an exemplary embodiment of the invention, the collected headerscomprise frame headers which indicate the frame type of thecorresponding frame (e.g., I, P or B) and timing information, such asthe presentation time stamp (PTS) and/or the decoding time stamp (DTS),which can be used to correlate between the frame headers and the framesin the video stream. Alternatively or additionally, other information,such as a unique ID is used to correlate between the frame headers andthe frames of the video stream.

The collected headers in this exemplary embodiment optionally furtherinclude macro-block headers which indicate for each macro-block the typeof the macro-block (i.e., intra or non-intra), a motion vector of themacro-block if any and the quantization of the macro-block.

Network

The principals of the present invention may be used with substantiallyany network type, including satellite, cable, Ethernet, fiber, cellularand combinations thereof. The communication between P&S unit 250 andtranscoder 210 is optionally performed over a standard communicationlink. Although P&S unit 250 and transcoder 210 may be at any distancefrom each other, in some embodiments of the invention, transcoder 210 isdistanced from P&S unit 250 by at least 100 meters, 500 meters or even10 kilometers. In some cases, transcoder 210 is distanced from P&S unit250 by at least 100 kilometers or even 1000 kilometers.

Encoding Engine

In some embodiments of the invention, encoding engine 124 is configuredwith default parameter values which are used whenever the informationfrom parameter selector 222 is not complete, for example due to errors,or due to deliberate dropping of headers which match the defaultparameter values, in order to reduce transmission overhead caused by theparameter values.

The encoded video stream from encoding engine 124 may be stored in afurther storage unit (not shown), for example to allow network personalvideo recorder (NPVR) services, without requiring repeated transcoding.

Alternatives

Rather than transmitting streams with parameter values of a singlesecond compression format, server 232 may transmit video streams withparameter values relating to a plurality of compression formats. Eachtranscoder 210 may then use the parameter values it requires for itstranscoding and ignore those it does not require. In some embodiments ofthe invention, a single transcoder 210 transcodes a single video streaminto a plurality of different compression formats for different clients112. Alternatively or additionally, transcoder 210 converts differentvideo streams received thereby into different compression formats.

Alternatively to transcoder 210 converting a compressed video streamfrom one compression type to another, transcoder 210 may use theparameter values from parameter selector 222 to convert a video streamfrom one bandwidth size to another, for example in order to fit on asmaller channel leading to a specific client 112. In some embodiments ofthe invention, transcoder 210 is utilized to down sample the videostream to a lower resolution, for example, from a high definition (HD)to a standard definition (SD) format. Optionally, in these embodiments,transcoder 210 down samples the video stream after the stream is decodedby decoder 120 and then encoding engine 124 encodes the down sampledstream. The parameter values received from parameter selector 222 maypertain to one or both of the down sampling and encoding.

Compression Methods

Although reference was made to the MPEG-2 and H.264 compression formats,the principles of the present invention may be used with substantiallyany other compression methods, including for example, Windows mediavideo (WMV9) compression, VC-9, common intermediate format (CIF), QCIFand Dirac compression methods. In some embodiments of the invention,video streams in the first compression format are more compact thanvideo streams in the second compression format, so that the transmissionover network 108 requires minimal bandwidth.

FIG. 4 is a block diagram of a video distribution system 400, inaccordance with another exemplary embodiment of the invention. FIG. 4 issimplified and shows only elements important for the followingdescription. System 400 differs from the embodiment of FIG. 2 in thatits transcoder 410 includes a partial parameter selector 402, whichselects some of the parameters of the compression using any method knownin the art. Nevertheless, the values of one or more parameters, aredetermined in advance by parameter selector 422. For example, parameterselector 422 may select the frame types and motion vectors, whileselector 402 selects the quantization and macro-block type.

Further Transcoding

As mentioned above, in some embodiments of the invention, the transcodedstream from transcoder 210 is further transcoded by a rate transcoder240. Rate transcoder 240 may operate in accordance with substantiallyany method known in the art. In some embodiments of the invention, thevideo streams include auxiliary information to aid rate transcoder 240in performing its task. This auxiliary information is different from theparameter values used by transcoder 210. The auxiliary information mayinclude for example compression hints, as described for example in theabove mentioned US patent publication 2001/047517. Alternatively oradditionally, the auxiliary information comprises replacement blocks asdescribed, for example, in PCT application PCT/IL2005/001326, thedisclosure of which is incorporated herein by reference.

While transcoders 210 and 240 may be separate entities, possibly locatedin separate locations, transcoders 210 and 240 may alternatively beincluded in a single housing, for example may both be implemented bysoftware on a common processing unit.

Conclusion

It will be appreciated that the above described methods may be varied inmany ways, including, changing the order of steps, and/or performing aplurality of steps concurrently. It should also be appreciated that theabove described description of methods and apparatus are to beinterpreted as including apparatus for carrying out the methods andmethods of using the apparatus. The present invention has been describedusing non-limiting detailed descriptions of embodiments thereof that areprovided by way of example and are not intended to limit the scope ofthe invention. Many specific implementation details may be used.

It should be understood that features and/or steps described withrespect to one embodiment may be used with other embodiments and thatnot all embodiments of the invention have all of the features and/orsteps shown in a particular figure or described with respect to one ofthe embodiments. Variations of embodiments described will occur topersons of the art. Furthermore, the terms “comprise,” “include,” “have”and their conjugates, shall mean, when used in the claims, “includingbut not necessarily limited to.”

It is noted that some of the above described embodiments may describethe best mode contemplated by the inventors and therefore may includestructure, acts or details of structures and acts that may not beessential to the invention and which are described as examples.Structure and acts described herein are replaceable by equivalents whichperform the same function, even if the structure or acts are different,as known in the art. Therefore, the scope of the invention is limitedonly by the elements and limitations as used in the claims.

1. A method of delivering a video stream, comprising: encoding a videostream into a first encoded stream; selecting encoding parameter valuesfor subsequent transcoding the first encoded stream into a specificsecond encoded stream; transmitting the first encoded stream with theselected encoding parameter values to a transcoder over a standardcommunication network; transcoding the first encoded stream into asecond encoded stream having the selected parameter values; andforwarding the second encoded stream.
 2. A method according to claim 1,wherein transmitting the first encoded stream comprises transmitting toa transcoder over a standard communication network, over a distance ofat least 1 kilometer.
 3. A method according to claim 2, whereintransmitting the first encoded stream comprises transmitting to aplurality of transcoders.
 4. A method according to claim 1, whereintransmitting the first encoded stream comprises transmitting over anEthernet, IP cable or satellite network.
 5. A method according to claim1, wherein transcoding the first encoded stream into a second streamcomprises decoding the first encoded stream into a non-encoded streamand re-encoding the non-encoded stream into the second encoded stream.6. A method according to claim 1, wherein the second encoded stream islarger than the first encoded stream by at least 20%.
 7. A methodaccording to, claim 1, wherein the first and second encoded streamsdiffer in the resolution of the video stream.
 8. A method according to,claim 1, wherein the first and second encoded streams differ in theirencoding method.
 9. A method according to claim 8, wherein transcodingthe first encoded stream into a second encoded stream comprisestranscoding an H.264 stream into an MPEG-2 stream.
 10. A methodaccording to, claim 1, wherein transmitting the first encoded streamwith the selected encoding parameter values comprises transmitting withall parameter values required for generating the second encoded stream.11. A method according to, claim 1, wherein transmitting the firstencoded stream with the selected encoding parameter values comprisestransmitting with fewer than all the parameter values required forgenerating the second encoded stream.
 12. A method according to claim11, wherein transmitting the first encoded stream with the selectedencoding parameter values comprises transmitting with motion vectorvalues.
 13. A method according to, claim 1, wherein transmitting thefirst encoded stream with the selected encoding parameter valuescomprises transmitting values for one or more specific parameters, formore than 90% of the video stream.
 14. A method according to, claim 1,comprising transcoding the second encoded stream into a third encodedstream.
 15. A method according to claim 14, wherein transcoding thesecond encoded stream into a third encoded stream comprises transcodingafter the forwarding of the second stream.
 16. A method according toclaim 14, wherein transcoding the second encoded stream into a thirdencoded stream comprises decoding the second stream into a non-encodedstream and then encoding into the third encoded stream.
 17. A method ofdelivering a video stream, comprising: receiving, over a standardcommunication network, a first encoded video stream together withencoding parameter values; transcoding the first encoded stream into asecond encoded stream having the encoding parameter values, receivedwith the stream; and forwarding the second encoded stream.
 18. A methodaccording to claim 17, wherein receiving encoding parameter valuescomprises receiving values of at least one encoding parameter for atleast 95% of the video stream.
 19. A method according to claim 17,wherein receiving encoding parameter values comprises receiving valuesof at least three encoding parameters for at least 80% of the videostream.
 20. A method according to claim 17, wherein receiving encodingparameter values comprises receiving motion vector values.
 21. Atranscoder, comprising: a network interface; a processor configured toreceive an encoded video stream through the network interface, togetherwith encoding parameter values and to transcode the received encodedvideo stream into a different encoded video stream having the receivedencoding parameter values.
 22. A transcoder according to claim 21,wherein the processor is configured to decode the received encoded videostream into a non-compressed video stream and to re-encode thenon-compressed video stream in transcoding the received encoded videostream.
 23. A transcoder according to claim 21, wherein the processor isconfigured to generate the different encoded video stream substantiallyentirely according to received encoding parameter values and withoutselection of parameter values.
 24. A transcoder according to claim 21,wherein the processor is configured to generate the different encodedvideo stream with received parameter values of a plurality of differentparameters for at least 90% of the video stream.
 25. A video server,comprising: an input interface for receiving video streams; an encoderconfigured to compress video streams received through the inputinterface according to a first encoding format; a parameter selectorconfigured to select values for one or more parameters of a secondencoding format, for the received video streams; and an output interfaceconfigured to transmit video streams compressed by the encoder withcorresponding parameter values selected by the parameter selector,toward a transcoder.
 26. A video server according to claim 25, whereinthe parameter selector is configured to select only a single value forat least 95% of the parameter instances for which it selects values. 27.A video server according to claim 26, wherein the parameter selector isconfigured to select only a single value for at least 99% of theparameter instances for which it selects values.
 28. A video serveraccording to claim 25, wherein the output interface is configured totransmit compressed video streams with corresponding parameter values,including only a single value for at least one encoding parameter, forat least 99% of the video stream.
 29. A method of transcoding a videostream, comprising: receiving a video stream in a first format;receiving motion vector values for transcoding the video stream into asecond format; and transcoding the received video stream into the secondformat using the received motion vector values.
 30. A method accordingto claim 29, wherein receiving the motion vector values comprisesreceiving together with the video stream.
 31. A method according toclaim 29, wherein receiving the motion vector values comprises receivingthe motion vector values and the video stream separately.